I. Field of the Invention
The present invention relates to a method and an apparatus for treating steel plate structures, especially steel sheets of automobile bodies or the like.
II. Description of the Prior Art
Chemical conversion treatment with phosphate is generally performed to treat steel sheet structures such as automobile bodies. Typically known chemical conversion treatments with phosphate are then immersion method and the spray method. These methods have advantages and disadvantages. For treating objects which are manufactured on automatic manufacturing lines, such as automobile bodies, the spray method is widely adopted due to stable productivity and quality.
Recently, in snowy areas such as the North American and European countries, a deicing agent such as rock salt or a mixture of rock salt with sand is scattered over the surface of the road. Owing to this, the outer panels of the automobile body, wheel houses and sills, and the interior of the doors are significantly corroded by the salt components, and improvements in resistance to scab corrosion and perforation corrosion have been desired.
As for the outer panels of the automobile body, these resistances have been significantly improved by improving the composition of the phosphate treatment solution mainly to achieve a lower zinc content (Zn/PO.sub.4 =1/12 or less) and a suitable total paint film thickness. However, the treatment solution may not be directly sprayed on the inner panel of an automobile body of complicated sections. Even if the treatment solution is directly sprayed on the inner panel, satisfactory degreasing during the cleaning stage and satisfactory formation of the chemical conversion coating may not be accomplished although it is not clear whether this is attributable to short treatment time or an insufficient amount of the treatment solution.
With the immersion method, the treatment time is 300 seconds or longer in comparison to 90 to 150 seconds with the spray method. The weight per unit area of the conversion coating is also as high as 5 g/m.sup.2 in contrast with 1.5 to 2.5 g/m.sup.2. The adhesion of the paint film to a coating to be formed in the process to follow is also inferior.
In order to shorten the treatment time in the immersion method, to make the conversion coating weight only 2.0 to 3.5 g/m.sup.2, and to obtain a phosphate chemical conversion coating with good adhesion to the paint film to be formed thereon, it is necessary to control well the composition of the phosphate treatment solution and the conditions for the immersion treatment. For example, it is shown in the literature that the characteristics of the zinc phosphate coating of excellent quality formed by the immersion treatment method are within the range shown in Table 1 below:
TABLE 1 ______________________________________ Characteristics of Zinc Phosphate Coating Item Characteristic ______________________________________ Coating Weight 2.5 to 3.8 g/m.sup.2 P/(P + H) 0.9 to 1.0 Crystal Size 3.5 to 10.mu. Crystal Shape Columnar or Granular ______________________________________ (Note) (1) P/(P + H): P represents phosphophyllite Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O, and H represents hopeite Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O, which are included in the coating of zinc phosphate. The characteristic shown in Table 1 above represents the content ratio of phosphophyllite. (2) Crystal size: represents the maximum length of the crystal. (3) Crystal shape: the zinc phosphate coating formed by the spray method and the conventional immersion method has a pointed, needlelike crystal shape.
Even if the composition and the temperature of the treatment solution in the immersion treatment are controlled, the characteristics of the obtained coating vary depending upon the immersion conditions.
In the phosphate conversion coating treatment of a steel sheet structure by immersion, in order to form a good and uniform phosphate coating suitable as a substrate for electrodeposition painting, it is desirable to make flow previously a liquid layer at a given depth below the surface of solution in the vicinity of the part wherefrom the steel sheet structure enters a tank of conversion coating treatment solution until a flow velocity of 20 m/min, preferably 10 m/min, is attained and to maintain the passing time of a given point of the body of steel sheet structure at 15 sec. or less, preferably 10 sec. or less.
Even when the flow velocity of the liquid layer is 20 m/min or more and the passing time is 15 sec. or more, an even phosphate conversion coating is formed in the same way. The phosphate coating thus formed, however, is of low quality and unsuitable as a substrate for painting by electrodeposition, because in the characteristics of coating, the crystal size becomes more than 15.mu. and the crystal form, needlelike, and because further the ratio P/P+H becomes 0.8 or less.
Moreover, in the phosphate conversion coating treatment of a steel sheet structure, there tend to occur mainly in the inside surface of the steel sheet structure portions such as air and gases formed by chemical reactions are stagnant and slowly move. Therefore, the formation of phosphate coating in these portions is remarkably hindered by such a tendency, and the hindrance constitutes a factor to form rust and blue color (iron phosphate). Accordingly, the parts wherein the formation of conversion coating is insufficient clearly show a poor adhesion of paint film and low corrosion resistance after painting.
Therefore, in order to form an excellent phosphate conversion coating, it is necessary to make easy the dispersion and transfer of the gases produced by the chemical reactions away from the steel sheet structure.
In addition to this, in the chemical conversion treatment with phosphate, the sludge formed by the reaction settles on the horizontal part or the box section of the object to be treated, and is held in the pinholes between the crystals of the coating or is deposited on the surface thereof. Even if the treated object is sprayed in the rinsing step to follow, the sludge may not be removed with ease. This leads to defective electrodeposition which is to be performed later, and degrades the qualities of the paint film, such as the paint adhesion, water resistance, and blister resistance. In order to solve these problems, it is necessary to change the shape of the bottom of the treatment bath tank and to reduce the amount of sludge in the treatment bath.